Brainteaser Alert! How many urban public toilets are there, globally? how many are needed by 2030?

Greetings. I'm working to develop high level, global estimates for the # of urban public toilets in the world, as well as estimate how many are needed, for use in global advocacy efforts around public, community, and shared toilets. I'm currently looking at different datasets that can be used to estimate today's coverage (e.g. on urban population, growth rates, density, sanitation coverage), and am looking at conditionalities to model what might be needed (e.g. the effect of increased hh sanitation on public toilet use).

I'm looking for examples and case studies from around the world that illustrate the assumptions for public toilets as they are planned and built, with preference for lower income countries. What's the ideal population density needed to support a public toilet in your city? Are the assumptions different based on male/female users? How are public toilets differentiated from businesses being required to provide toilets on site? How do mosques and other religious centers plan for the # of toilets to meet their variable demand?

If the brainteaser compels you, what information and assumptions do you think are essential to answer the question?